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Lightweight Design worldwide

, Volume 10, Issue 5, pp 20–25 | Cite as

Specific challenges in the application of fibre-metal laminates

  • Daniel Stefaniak
  • Robert Prussak
  • Lennart Weiß
Materials Fibre-Metal Laminates

Fibre-metal laminates as a combination of metals and fibre reinforced plastic materials are investigated in a variety of current research projects. The intention of combining these two different materials is the compensation of their inherent weaknesses. However, their application as multi-layered laminate is also accompanied by a wide range of challenges, as DLR and TU Braunschweig scientists describe.

Intrinsic Hybrid Laminate

Fibre reinforced plastic (FRP) composites meet the requirements of many different lightweight applications due to their extraordinary weight-specific stiffness and strength as well as their marginal fatigue. However, they also show some major weaknesses like low bearing strength or brittle failure properties. Fibre-metal laminates (FML), consisting of fibre reinforced plastic and metal, are often investigated with the aim to overcome these weaknesses. Although many different definitions for FML can be found, the major difference to hybrid structures or composite constructions is that the material combination affects the laminate architecture. This means that an FML consists of at least one FRP and one metal layer. Additionally, the term ’intrinsic hybrid laminate’ has been established to describe those FML where the cohesion between FRP and metal is created by the FRP matrix during its cure without the use of any additional adhesive.

Unfortunately, it can be observed that FML are taken into consideration for former metal parts which are made of FRP without changing their geometry. This approach is strikingly called ’black metal’ design as carbon fibre reinforced plastic (CFRP) is used in the geometry of a metallic part instead of adapting the design to the specific demands of FRP with regard to mechanics and manufacturing. Intrinsic hybrid laminates are than employed to compensate the inappropriate design.

The selection of the FML based on the desired properties and a subsequent detailed design is more adequate and recommended. However, it must be considered that the material properties of an FML do not depend on the material combination only. The material fractions and in particular the layer thickness of both constituents are of major influence as well.

The best known FML consists of glass fibre reinforced plastic (GFRP) and aluminium: Glare. The purpose of the GFRP layers is to bridge fatigue cracks induced in the aluminium and to reduce their progress [1]. Therefore, parts under cyclic tensile load, like the upper central fuselage panels of the Airbus A380, are preferred for its application. The glass fibres in Glare are preferably oriented along the main load direction or as a bidirectional laminate.

The selection of the FML based on the desired properties and a subsequent detailed design is more adequate and recommended.

A further application for ’intrinsic hybrids’ are UD-CFRP-steel laminates. Especially, when high specific uniaxial mechanical properties are aspired, notch and impact sensitivity properties limit the fibre fraction in load direction drastically. Therefore, the laminates loose a certain fraction of their lightweight potential mainly due to the residual strength considerations. Recognising these limitations, UD-CFRP-steel laminates aim to reduce the aforementioned disadvantages by adding steel layers with a thickness below 0.08 mm up to a maximum metal volume fraction (MVF) of 12 % steel. These layers do not reduce stiffness and strength. They even increase the residual strength of previously impacted laminates. Tests show an increase of weight-specific stiffness and strength up to 15 % at a comparable residual compression strength after impact level [2]. At the same time, the weight-specific residual compression strength of a laminate with identical damage size is increased significantly.

One major challenge in the design of FRP parts is the brittle nature of the material, whereas the integration of metallic layers allows an adjustment of the failure behavior. In this way, by adjusting the hybrid material, a desired load and deformation behavior of a part exposed to a crash load scenario, is achievable without changing its geometry. Figure 1 presents the load-displacement of three cylindrical specimens of different materials. Figure 2 shows their arrangement in a 3-point-bending static test as used for drop weight tests accordingly. The specimens are made from CFRP, HSD-steel or a combination of both. The load-displacement curve of the FML may be adjusted by varying lay-up and MVF. By doing so, comparably high specific energy absorption is achievable.
Figure 1

Force-displacement curve of comparable cylindrical tubes made of CFRP and HSD steel as well as their combination in a hybrid material system (© DLR)

Figure 2

3-Point-bending setup for investigations on cylindrical tubes: Picture shows static testing, setup used for drop tests accordingly (© DLR)

The creation of an FML can also be limited to a certain area of a part by adding FRP locally to a metallic part or by adding metal locally to an FRP part. The material’s exploitation of FRP in a part, especially when using CFRP, is often limited by its low bearing strength and it may only be exploited to its full extent when adding a ramp-up to the part. By adding metallic inserts, the laminate’s bearing strength increases and a local FML is created. An even higher load carrying capacity is achieved when multiple metallic layers are used [3]. In this case, FRP-layers are substituted by titanium or stainless steel sheets, whereby three different areas are created: a pure FRP area, a hybrid region with the maximum metal content and a transition region in between the first two areas. With the help of a certain metal content and a structural design of the transition region, the load bearing capacity can be increased in a way that no additional ramp-up is required [3, 4, 5].

An intrinsic hybrid is also created by using only one metal layer, for example adding a metallic abrasion protection layer to the outer surface of an FRP part [6] or integrating metallic conductive tracks in the FRP laminate.

Material Selection

’Intrinsic hybrid laminates’ offer a variety of parameters with regard to their lay-up. Besides the material of the two constituents, their content, single layer thickness and their arrangement are variable. The metal volume fraction (MVF) is widely used to describe the material content.

A simple analytic estimation of stiffness and strength depending on the MVF can be done by using rule of mixtures or classical laminate theory (CLT). The first method does not consider the suppression of the transverse contraction evoked by adjacent layers, and hence, is not recommended for low MVF [7].

Thereby, the weight-specific stiffness and the compression strength of certain material combinations can be calculated with low effort, as exemplary shown in

Figure 3 and Figure 4 for steel 1.4310, titanium 15-3-3-3 and aluminium 7075 in combination with two common prepreg-systems 8552/AS4 (HTS) and 8552/IM7 (IM). Both CFRP systems utilise the same matrix but different fibres with the IM fibre having higher strength and stiffness. The metal’s yield strength is relevant in most material combinations, as the failure strain of the fibres is larger than the elastic strain of the metals in the majority of cases. Hence, the strength depends on stiffness, MVF, coefficient of thermal expansion (CTE) and the difference between cure and operational temperature [2]. Figure 3 shows the weight-specific stiffness of steel, titanium and aluminium in combination with both CFRP types. At 0 % MVF, pure CFRP properties are achieved. Accordingly, all graphs meet at almost one point at 100 % MVF, as the three metals have similar weight-specific stiffness. The residual thermal stresses are considered in addition when regarding the laminate’s yield strength at room temperature in Figure 4. A so called ’stress free temperature’ of 180 °C is supposed (166–168 °C meet the standard curing process of the prepreg [8]) here. The residual stresses have a favorable impact on the regarded compression strength in Figure 4 compared to those cases where residual stresses are not considered (grey graphs). The comparably high weight-specific strength of CFRP can only be maintained by using low fractions of steel.
Figure 3

Weight-specific elastic modulus dependency on metal volume fraction for steel, titan and aluminium combined with HTS- and IM-fibre reinforced plastic (© DLR)

Figure 4

Compressive yield strength dependency on metal volume fraction with and without (NR) consideration of residual stresses for IM-fibre and strength of pure IM-fibre CFRP (© DLR)

Some essential characteristics of the material combinations can be derived by such a simple approach, as Figure 4 for example, indicates that a high laminate compression strength increase is received as a consequence of residual stress for the combination of CFRP and aluminium at low MVF. However, these stresses lead to a significant decrease in tensile strength and prohibit a reasonable use of this material combination.

FML Manufacturing

The design of the interface between matrix and metal surface has a large impact on the material’s behavior. In addition, the metal layers serve as a barrier layer against fluids. Although prepregs are used mostly for the manufacturing of intrinsic hybrid laminates, depending on the required flow distance, resin infusion techniques are applicable as well.

The surface treatment of the metal foil is one crucial process step in the FML manufacturing process. Much research has been conducted, focusing on the pre- and post-treatment of aluminium and titanium surfaces, but, only few investigations dealt with stainless steel. Therefore, different pickling and blasting processes were investigated and a vacuum blasting process for coiled material has been developed. These surface treatment processes are compared with the ’Boeing sol-gel process’ as reference. Figure 5 shows the developed vacuum blasting machine where the grit is guided contrariwise through two nozzles. The coiled metal sheet is conveyed continuously through both nozzles and the sheet is grit blasted simultaneously on both surfaces.
Figure 5

Vacuum blasting unit with peripherals (© DLR)

The development of the surface treatment is essentially performed empirically and hence, requires the evaluation of the adhesion. In general, this evaluation is done by determining the interlaminar shear strength with the help of the 3-point-bending test, the 5-point-bending test or the double-lap shear test. An alternative is to use the crack energy release rate for evaluation. The main disadvantage of most of the specimen geometries is that residual thermal stresses superimpose the stresses generated by the test load and falsify the results. The mentioned bending tests show an advantage as the larger residual shear stresses in the specimen`s longitudinal direction do not act in the loaded area [2]. However, when applying DIN EN ISO 14130 to determine the interlaminar shear strength, the special characteristic of the shear stress in FML needs to be taken into account, which is a consequence of the different single layer stiffnesses. In addition, testing at different temperatures and different moisture contents is recommended.

For the combination of steel 1.4310 and thermoset CFRP, the vacuum blasting process achieved significantly higher interlaminar shear strength than the reference and proved to show high potential for automation [9].

Thermal Residual Stresses

Residual thermal stresses occur as a consequence of the difference in CTE of the two constituents of an ’intrinsic hybrid’ and the difference between cure temperature of the matrix and operational temperature of the cured laminate. In non-symmetrical laminate lay-ups, inhomogeneous residual stresses are generated which result in deformations. As further effects occur as well and may lead to thermal stresses and deformations, they must be differentiated systematically to develop analytical and numerical methods for their evaluation.

By introducing a cooling step the residual thermal stresses can be reduced by approximately 25 %.

Different measuring methods are applicable to analyse influential parameters: tensile testing, warping of non-symmetrical bi-material strips, Figure 6, use of embedded fibre Bragg (FBG) sensors or strain gauges [8].
Figure 6

Symmetrical bi-material strips with integrated FBG sensors and separated non-symmetric specimens (© DLR)

A combined approach using FBG sensors and warping evaluation of non-symmetric strips has been developed at the DLR. Non-symmetric specimens are created by removing certain layers after cure, Figure 6. A quantitative evaluation is achieved by determination of their curvature and measuring the FBG strain at room temperature [10].

In order to develop special cure cycles to lower the residual stresses, the simultaneous measurement of the strains during cure is beneficial [8]. This is also done with FBG sensors, which consist of a glass fibre with Bragg gratings, embedded in the laminate, Figure 7, and allowing the strain measurement during and after cure. The required temperature compensation is done with the help of additional thermocouples.
Figure 7

Embedded FBG sensor in a CFRP-steel-laminate, identification of the Bragg grating by laser (© DLR)

By introducing a cooling step in the curing cycle and taking advantage of the exothermal cross-linking reaction`s inertia, the residual thermal stresses in an FML using the above mentioned matrix system can be reduced by approximately 25 %.

Nondestructive Testing

The application of nondestructive test methods on a material is a crucial requirement for its use. In aerospace, the inspection is performed visually with or without any auxiliaries or ultrasonic. Computer tomography (CT) is applied for very few more detailed inspections.

The visual detection of impacted parts is simplified by the use of FML as, in contrast to pure CFRP, failures in the laminate are indicated by a detectable failure on the laminate surface. However, due to the large number of layers with different acoustic impedances, ultrasonic inspection by pulse-echo is only suitable to a limited extent. CT however, is also accompanied by severe streaking artifacts that occur due to the fact that the high density of the metal results in incomplete attenuation profiles. This overranging can be reduced by means of special software corrections, but a loss of detail around the metal interface remains, even though different physical filters have been tested. As a result, the effort to characterise the failure’s geometry is increased and impedes the better understanding of the involved failure mechanisms.

When combining ultrasonic and CT inspection, it has to be taken into account that both indicate different failure types in particular [2]. Figure 8 depicts the results of an ultrasonic scan, of a CT as a projected top view and of CT as a cross-section of the same specimen. The cracks apparent in the cross-section are directly assigned to the CT top view. Comparing the pictures, it also becomes clear that the ultrasonic inspection detects significantly larger failures in the form of delamination than the CT. The cracks apparent in the CT however, are longer than the delamination in the ultrasonic picture. Hence, a more detailed understanding of the failure mechanisms involved is only achievable by a combined use of both inspection methods.
Figure 8

Projected top view by CT and ultrasonic scan as well as CT cross-section of the same impacted CFRP-steel specimens (© DLR)

Summary

Different advantages of FML have already been shown in single applications. However, instead of using FML as a subsequent reinforcement of CFRP parts, they must be regarded as an individual material system to exploit their full potential. With regard to the variety of different fields of applications, the authors identified comprehensive research demand in the fields of the interface’s long-term durability, corrosion behavior, thermo-elastic properties, as well as in context of dimensional conformence.

Notes

Thanks

The presented findings were essentially gained during the project no. 8 of the ’Schwerpunktprogramm SPP1712’ funded by the ’Deutsche Forschungsgemeinschaft’. The authors like to thank for funding and support. Another word of thanks is addressed to Salzgitter Mannesmann Forschung for providing material.

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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Daniel Stefaniak
    • 1
  • Robert Prussak
    • 2
  • Lennart Weiß
    • 3
  1. 1.German Aerospace Center (DLR)StadeGermany
  2. 2.Technische Universität BraunschweigGermany
  3. 3.German Aerospace Center (DLR)BraunschweigGermany

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